The Frankenstein Hybrid
Jerry Flint, 05.30.06, 6:00 AM ET

New York - I've lived through all the hype about automotive miracles, such as the 100-miles-per-gallon carburetor, the modern steam car, the modern electric car, fuel cells and hydrogen cars. Billions of dollars have been spent searching for a car that doesn't use gasoline, or at least much gasoline.

This brings me to the latest bit of wizardry that some are promoting as an answer to all our problems: the plug-in hybrid.

Gas-electric hybrids have been on the market for several years. A hybrid is a vehicle with two propulsion systems: one regular internal-combustion motor and one or more battery-powered electric motors, which can drive the car for short distances or give the internal-combustion engine an assist during acceleration.

Now there's talk of something else, a plug-in hybrid. The idea is to give the vehicle additional battery power, which will allow it to run longer and farther on less gasoline. Today's Toyota Motor (nyse: TM - news - people ) Prius hybrid starts out under battery power only but switches to its gasoline engine when it reaches 15 to 20 miles per hour. A plug-in, on the other hand, might run 20 or more miles on battery power alone. Of course, to get this extra range requires additional batteries, which drivers can recharge off the electric grid. Hence the name "plug-in."

Supporters of plug-in hybrids talk of 100 miles per gallon and batteries that are rechargeable at home. No car company currently makes a plug-in, but innovators have put some together and received great publicity. Forbes was the first publication I read that cheered the plug-in development. Tom Friedman of The New York Times began writing columns about the plug-in saving us from our oil troubles, and a small cult has developed around the concept.

Toyota knows more about hybrids than anyone does, so I asked about plug-ins. Here is the official statement: "With today's latest technology, a plug-in HV [hybrid vehicle] is not commercially and technologically feasible." I have found Toyota's word is golden on stuff like this.

The problem is batteries--they are why the dream of a purely electric car has yet to come to fruition. Today's battery technology cannot give the necessary power and range to make batteries practical for the average consumer. They also add weight and are costly.

Hybrids are different from electric cars. Most hybrids use nickel-metal-hydride batteries, which last for many years--perhaps as long as the car. Toyota tells me there are Priuses that have gone more than 275,000 miles on the original batteries. The secret to such longevity? According to Toyota, the car never charges its batteries to more than 80% of their capacity nor drains them below 50% capacity.

The philosophy behind a plug-in hybrid is that it drains the battery to almost empty before the gas engine cuts in. That is how they make those 100-mpg claims. This requires a bigger battery pack and, preferably, a lithium-ion battery, which packs more power per pound than nickel metal hydride.

Wade Hoyt, manager of Toyota's Northeast public relations and a Popular Mechanics veteran, tells me, "In typical consumer products [digital cameras, cell phones, etc.], the batteries are charged to 100% of capacity most times, then drained to 20% or less. This wears [the batteries] out."

"The problem with lithium-ion batteries is that they generate a lot of heat. That's OK in a cell phone with a pretty small battery. But with a trunk full of batteries, you have a serious cooling issue. So until we find a battery that runs cooler and last longer with big swings in its state of charge, we don't think plug-in hybrids are commercially viable," says Hoyt. Remember, too, that the electricity coming to one's home is not pollution-free.

Toyota expects to sell 150,000 to 160,000 hybrids in the U.S. this year, and it designed all those systems to last more than 150,000 miles, or the life of the vehicle. That is quite different from jury-rigging a one-off science experiment and converting a Prius into a plug-in hybrid. Toyota engineers are investigating plug-ins as well as many other technologies. But keep in mind that Toyota has no stake in helping the oil companies boost demand for gasoline.

Since the dawn of the automotive age, car owners have modified their cars to make them go faster, handle better and look more distinctive than standard factory-built cars. But such alterations can take a toll on ride, handling and durability. Consumer who converts their hybrids into plug-ins risk voiding their warranties. One thing no one would want to do is burn out the Prius battery, as a replacement is likely to cost several thousand dollars.

The Toyota Prius is in short supply, but Honda Motor (nyse: HMC - news - people ) has had less success selling its Accord hybrid. With several more hybrids about to come onto the market, we will soon see whether there really is a demand from more than just the techies and early adapters, for these vehicles.

Someday plug-in hybrids may become commercially viable. But not now.

Jerry Flint, a former Forbes senior editor, has covered the automobile industry since 1958. Visit his homepage at www.forbes.com/flint .